Polymer spherulites: A critical review

Polymeric and non-polymeric materials often crystallize as spherulites when crystallized from viscous melts or solutions at large undercooling. The essential component of a spherulite is fibrillar crystals that grow in predominantly radial directions and branch irregularly. We review the growth, branching and twisting of crystals in the light of theoretical and experimental advances of the last decade, while maintaining an appreciation for historical context.The crucial role of self-generated fields ahead of the crystal–melt interface is developed. Pressure gradients from volume contraction have been treated, as well as impurity gradients ahead of a growing crystal; fibril width W is predicted and found to be proportional to δ^1/2, where the diffusion length δ = D/G, the quotient of diffusivity and growth rate, conveys the extent of the field gradient. Ribbon-like spherulite radii grow at a constant rate under diffusion-coupled interface control.Non-crystallographic branching is required to maintain the volume occupied by fibrillar crystals as the spherulite radius increases. Topological giant screw dislocations and induced nucleation at cilia tethered to crystals are observed mechanisms leading to branching normal to the wide dimension of lamellar crystals; but the relative importance of each of these is not yet established. Repetitive tip splitting by kinetic interface instability has been suggested as a branching mechanism in the wide dimension of lamellar crystals.Larger molecular mass reduces the spherulite growth rate, more so at low undercoolings, for reasons that remain unresolved. Miscible diluents often profoundly reduce G by lowering both thermodynamic driving force and local transport dynamics that govern the secondary nucleation rate. Spherulite blend morphology is linked to the competition between radial growth rate G and diffusivity D of the diluent, expressed as the diffusion length δ.Polymer crystals in which chain helices all have the same sense show banded spherulites, as do crystals in which the chain axes are not perpendicular to the basal surfaces. Recent analyses with optical birefringence and X-ray micro-diffraction support the presence of helicoidally twisted ribbons, although other structural arrangements have sometimes been revealed by microscopy. Assessments of twist directions in spherulites of chiral polymers point to unbalanced basal surface stress as the source of twisting, although a general mechanical analysis is lacking. Another twisting model employs regular arrays of isochiral giant screw dislocations; results are mixed for this model.     

Six types of spherulite morphologies with polymorphic crystals in poly(heptamethylene terephthalate)

Six types of spherulite morphologies packed with polymorphic crystals and their growth kinetics in melt-crystallized poly(heptamethylene terephthalate) (PHepT) were characterized using polarized-light optical microscopy (POM), Fourier transformed infrared microspectrometry (micro-FTIR), differential scanning calorimetry (DSC) and atomic-force microscopy (AFM). Two maximum melting temperatures (T_max), a higher 150 °C and a lower 110 °C, were used to melt the initially crystallized PHepT of either α- or β-crystal. The high T_max was enough to melt all nuclei, but the lower T_max was considered as near or slightly below the equilibrium melting temperatures of these two cells (if estimated by nonlinear methods). When crystallized at various T_c from these two T_max’s, PHepT can exhibit as many as six types of spherulites (Ring Type-I, -II, -III, Maltese-cross Type-1, -2, and -3) owing to different nucleations. Ring Type-I, Maltese-cross Type-1 and -3 spherulites are packed of the sole β-crystal, while Ring Type-II, -III and Maltese-cross Type-2 spherulites are attributed to the sole α-crystal. However, as the PHepT polymorphic cells are related to T_c, such correlations between the crystal cells and spherulite types (ring or ringless) cannot be ruled out to be a coincidence.     

Investigation of the inherent electrical potential of PEO electrolyte films

Measurement of the through-plane potential of PEO-lithium triflate electrolyte films has demonstrated that they possess an inherent potential as cast from an acetonitrile solution onto a Teflon substrate. These films have an inherent potential of around 0.2 V and the cast films display a discharge behavior similar to a double layer capacitor system with a small discharge capacitance of 80 nF cm⁻². It is postulated that electrochemical properties of the films can be attributed to different salt concentration at the two surfaces. This difference in concentration may result from a matching of the surface-free energy of the Teflon substrate side of the film and the side of the film where evaporation occurs with the lithium triflate species in the polymer. Different spherulite morphologies were also observed for each surface. These morphologies can be assigned to spherulites having much different ion concentrations. Attenuated total reflection (ATR) IR spectroscopy was used to investigate the surface concentrations of free ions, ion pairs and ion multiples of both surfaces of the films. AC impedance spectroscopy of the surfaces of the film was also conducted. These data indicated that there is a difference in the surface concentration of each side. The ability of electrolyte films to exhibit a potential as fabricated may have potential applications as an easily manufactured power source for micro and nanodevices.     

Morphology of a melt crystallized iPP/HDPE/hydrogenated hydrocarbon resin blend

The structure, phase structure, morphology, crystallization and melting behavior of isotactic polypropylene (iPP) blended with a master batch (MB), formed by high density polyethylene and hydrogenated hydrocarbon resin (iPP/MB), have been in details investigated by using X-ray diffraction, optical microscopy and differential scanning calorimetry. It was found that the structure and morphology depend on crystallization conditions. A new family of α spherulites of iPP (type I spherulites) can be activated using appropriate crystallization conditions. Nucleation of these spherulites has been explained by using the approach of nucleus migration in polymer blends. Type I spherulites present specific morphological, kinetic and thermal behaviors. In particular it was found that the growth rate of type I spherulites, at a given T_c, is higher than the growth rate of spherulites grown from plain iPP.     

Simultaneous liquid–liquid demixing and crystallization and its effect on the spherulite growth in poly(ethylene terephthalate)/poly(ether imide) blends

Poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blends were miscible in the melt, but exhibited simultaneous liquid–liquid phase separation and crystallization over a wide range of temperature and composition. The interplay between these two processes is expected to dominate the morphological formation in the blends. In this study, the phase diagram of PET/PEI blend was determined to evaluate the envelop within which liquid–liquid phase separation was operative with crystallization. A UCST phase diagram below 240°C was identified for this system. The effect of liquid–liquid phase separation on the growth of PET spherulites was studied by small-angle light scattering (SALS). Nonlinear spherulite growths were observed for the blends at higher crystallization temperatures of 210°C and 220°C, while the growths were basically linear below 210°C. The nonlinear growth behaviour was discussed based on the competition between spherulite growth and spinodal decomposition.     

Biomimetic Crystallization of Calcium Carbonate Spherules with Controlled Surface Structures and Sizes by Double‐Hydrophilic Block Copolymers

Big CaCO₃ spherules with controlled surface structures and sizes ranging from several hundreds of nanometers and micrometers can be easily fabricated through a slow gas–liquid diffusion reaction at room temperature by using double‐hydrophilic block copolymers (DHBCs) as crystal modifiers. The influence of the DHBCs with different functionalities such as carboxyl, partially phosphated, and phosphorylated groups on the crystallization and structure of calcite formation was investigated. The morphology evolution process and the early stage of the big spherical superstructures were followed. Such big spherules with complex surface structure made of calcite rhombohedra are not easily produced by conventional solution growth methods, and furthermore show high potential for chromatographic purposes due to their exposition of multiple calcite faces and the huge particle sizes suitable for chromatographic column packings.     

Polymer crystallization from the metastable melt: The formation mechanism of spherulites

One of the most popular crystalline morphologies is a spherulite. An evidence is reported that the spherulite is crystallized through a dense packing state of small particles appearing in a droplet, which is caused by the primary phase separation of the melt in the metastable region of a phase diagram proposed by Olmsted et al. [Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ. Phys Rev Lett 1998; 81: 373]. According to this phase diagram, the crystallization from the metastable state causes the nucleation and growth (N & G) of nematic domains, here named droplets, in the isotropic matrix. As a next step, the secondary phase separation of spinodal decomposition (SD) type into smectic and amorphous domains occurs inside the droplet where entanglements are excluded from the smectic to the amorphous domain; then such an SD structure turns into a densely packing structure of many small particles owing to surface tension. At this final stage of the induction period a long period peak of small-angle X-ray scattering (SAXS), so-called SAXS before WAXS, appears, which may be due to the average distance between these small particles. Furthermore, it is considered that crystalline lamellae are formed by radial and azimuthal fusion of these small particles inside the droplet, resulting in a spherulite. Such a type of crystallization occurs most commonly when flexible polymers are crystallized under the usual conditions. This tentative concept of spherulitic growth, which is completely different from a theory by Keith and Padden [Keith HD, Padden FJ. J Appl Phys 1963; 34: 2409], would give a new insight into problems of spherulites.     

Investigations on the enhancement mechanism of inorganic filler on ionic conductivity of PEO-based composite polymer electrolyte: The case of molecular sieves

Polarized optical microscopy (POM) and differential scanning calorimeter (DSC) techniques are used to study the effect of ZSM-5 molecular sieves on the crystallization mechanism of poly(ethylene oxide) (PEO) in composite polymer electrolyte. POM results show that ZSM-5 has great influence on both the nucleation stage and the growth stage of PEO spherulites. ZSM-5 particles can act as the nucleus of PEO spherulites and thus increase the amount of PEO spherulites. POM and DSC results show that ZSM-5 can restrain the recrystallize tendency of PEO chains through Lewis acid-base interactions and hence decrease the growth speed of PEO spherulites. Room temperature ionic conductivity of PEO-LiClO₄-based polymer electrolyte can be enhanced by more than two magnitudes during long time storage with the addition of ZSM-5.     

Thermal analysis, X-ray and electron diffraction studies on crystalline phase transitions in solvent-treated poly(hexamethylene terephthalate)

The crystal polymorphism, transformation, and morphologies in chloroform solvent-cast poly(hexamethylene terephthalate) (PHT) were examined by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and temperature in situ transmission electron microscopy (TEM). Solvent-induced crystallization of PHT at room temperature yielded an initial crystal of γ-form, as confirmed by WAXD. Upon DSC scanning, the original γ-form in PHT exhibited three endothermic peaks, whose origins and association were carefully analyzed. The first peak, much smaller than the other two, is in the temperature range of ca. 100-120 °C. It was found that the solvent-induced γ-form was transformed to β-form at 125 °C via a solid-to-solid transformation mechanism. In addition, WAXD showed that γ- and β-forms co-existed in the temperature range of 100-125 °C. These mixed crystal forms were further identified using TEM, and the selected-area electron diffraction (ED) patterns revealed that both γ- and β-form crystals co-existed and were packed within the same spherulite. Solid-solid transformation from the solvent-induced γ-form to β-form in PHT upon heat scanning was presented with evidence and discussed.